1. Statement of the Technical Field
The inventive arrangements concern wireless networks. More particularly, the invention relates to a system and method for the detection of wireless network nodes existing within a communications range.
2. Description of the Related Art
Wireless ad-hoc networks are known in the art. They generally include a class of networks in which wireless communications are used to link a plurality of nodes, which can be mobile. The term “node” as used herein refers to a device configured to establish a connection to another device in a wireless network. Such devices often include servers, handheld communications devices, mounted communications devices, sensor devices, relay devices, coordination devices, satellites and the like. Each node includes a transmitter and receiver. Significantly, each node can function as a router and/or as a host. In operation, each node is configured to communicate directly with other nodes (i.e. without the use of a centralized access point). The topology of the ad-hoc network is not fixed and various nodes automatically reconfigure themselves to function as routers on an as needed basis. Packetized data communicated in the network can travel from a source node to a destination node either directly, or through some set of intermediate packet forwarding nodes.
In order for an ad-hoc network to function properly, the nodes must be able to identify the presence of neighboring nodes in the network. Consequently, nodes are typically configured to execute a defined procedure to locate unconnected nodes in the network and determine paths through the network through which data traffic can be communicated from a source node to a destination node. These procedures can also be useful for detecting the departure of nodes from the network and identifying alternative paths through the network when such departures do occur.
One common method for identifying neighbor nodes involves the use of “beacons”. The term “beacon” as used herein refers to any wireless communication that is generated by a network node, and which can be potentially received by some or all nodes comprising the network for purposes of initiating discovery of unconnected nodes in the network. Thus, the beacon transmission is not necessarily intended to be received by any particular node, but can instead be selected so it can be received by any node within transmission range of the source node. Various types of information are advantageously included within the beacon signal. For example, the beacon signal can identify the source of the beacon signal by means of some identification code, can identify transmission and receive frequencies to be used, and various other communication protocols.
Various different types of beacon systems have been devised for use in ad-hoc networks. Some systems use only a limited number of selected nodes in the network to perform the beacon function whereas in other systems, all nodes can transmit a beacon. Some systems transmit a beacon signal in an omni-directional pattern to communicate with nodes in all directions. Other systems use a sectorized approach to selectively transmit a beacon signal only in pre-defined directions. When a beacon signal is received by a neighboring node, the beacon indicates to the receiving node that a neighbor node is present. Still other networks are designed to use a periphery-based approach where designated nodes located near peripheral areas of the network are selected to perform network acquisition processing for the entire network.
Neighbor discovery in an ad-hoc network is a key step to establishing network communications. Accordingly, beacon signals must be sent in such a way as to ensure a high likelihood that they will be received by other nodes in the network. In this regard, it is often desirable to transmit beacon signals at well defined intervals and relatively high power levels. Consequently, the beacon transmissions are very much subject to detection and interception by adversaries. Notably, in a combat environment, an adversary can exploit this information in a variety of ways. Therefore, a need exists for systems and methods to enable nodes to perform neighbor discovery with a low probability of exploitation.
One method for minimizing detection and interception of beacon signals involves the use of spreading sequences, which distribute the energy of the beacon signal over a wide bandwidth. Such a system is described in U.S. Pat. No. 7,269,198 to Elliott et al. The system described therein transmits a spread spectrum signal comprised exclusively of a spreading sequence, without any associated beacon data. The spreading sequence is transmitted at very low power to avoid detection and/or interception of the beacon signal by adversaries. However, the low power of the beacon signal means that, in order to be received by neighboring nodes, substantial amounts of processing gain is needed. This processing gain is achieved using “relatively long” spreading sequences. The large amounts of processing gain are advantageous, but necessarily limit the amount of data that can be transmitted. This is due to the fact that the data rate must be much lower than chipping rate that is associated with the spreading sequence. Accordingly, there remains a need for discovering neighbor nodes in ad-hoc wireless networks using beacon signals that have a low probability of detection/interception, but which also have the ability to communicate sufficient amounts of network data to immediately initiate communications. In this regard, detectability is further reduced by increasing the probability that any given neighbor discovery message will be properly received by an intended receiver, limiting the number of transmissions required to initiate communications.
Chaotic systems can generally be thought of as systems which vary unpredictably unless all of its properties are known. When measured or observed, chaotic systems do not reveal any discernible regularity or order. Chaotic systems are distinguished by a sensitive dependence on a set of initial conditions and by having an evolution through time and space that appears to be quite random. However, despite its “random” appearance, chaos is a deterministic evolution.
Practically speaking, chaotic signals are extracted from chaotic systems and have random-like, non-periodic properties that are generated deterministically and are distinguishable from pseudo-random signals generated using conventional PRNG devices. In general, a chaotic sequence is one in which the sequence is empirically indistinguishable from true randomness absent some knowledge regarding the algorithm which is generating the chaos.
Some have proposed the use of multiple pseudo-random number generators to generate a digital chaotic-like sequence. However, such systems only produce more complex pseudo-random number sequences that possess all pseudo-random artifacts and no chaotic properties. While certain polynomials can generate chaotic behavior, it is commonly held that arithmetic required to generate sufficiently large chaotic number sequences requires an impractical implementation due to the precision required.
Communications systems utilizing chaotic sequences offer promise for being the basis of a next generation of low probability of intercept (LPI) waveforms, low probability of detection (LPD) waveforms, and secure waveforms. While many such communications systems have been developed for generating chaotically modulated waveforms, such communications systems suffer from low throughput. The term “throughput” as used herein refers to the amount of data transmitted over a data link during a specific amount of time. This throughput limitation stems from the fact that a chaotic signal is produced by means of a chaotic analog circuit subject to drift.
The throughput limitation with chaos based communication systems can be traced to the way in which chaos generators have been implemented. Chaos generators have been conventionally constructed using analog chaotic circuits. The reason for reliance on analog circuits for this task has been the widely held conventional belief that efficient digital generation of chaos is impossible. Notwithstanding the apparent necessity of using analog type chaos generators, that approach has not been without problems. For example, analog chaos generator circuits are known to drift over time. The term “drift” as used herein refers to a slow long term variation in one or more parameters of a circuit. The problem with such analog circuits is that the inherent drift forces the requirement that state information must be constantly transferred over a communication channel to keep a transmitter and receiver adequately synchronized.
The transmitter and receiver in coherent chaos based communication systems are synchronized by periodically exchanging state information over a data link. Such a synchronization process offers diminishing return because state information must be exchanged more often between the transmitter and the receiver to obtain a high data rate. This high data rate results in a faster relative drift. In effect, state information must be exchanged at an increased rate between the transmitter and receiver to counteract the faster relative drift. Although some analog chaotic communications systems employ a relatively efficient synchronization process, these chaotic communications systems still suffer from low throughput.
The alternative to date has been to implement non-coherent chaotic waveforms. However, non-coherent waveform based communication systems suffer from reduced throughput and error rate performance. In this context, the phrase “non-coherent waveform” means that the receiver is not required to reproduce any synchronized copy of the chaotic signals that have been generated in the transmitter. Further, many non-coherent chaotic waveforms embed additional information in the signal that may be exploited by an unintended receiver to gain partial information of the transmission. The phrase “communications using a coherent waveform” means that the receiver is required to reproduce a synchronized copy of the chaotic signals that have been generated in the transmitter.